RonP

As I see it, multiple issues come into play.
IHD is presumptive (if you meet the A.O. related requirements) You evidently did.
PTSD requirements have also been liberalized, and you are currently rated at 50%, but need to apply for an increased rating for PTSD, based upon treatment records, etc.
Depending upon various considerations, timing etc. asking for reconsideration with "new" evidence, or filing an NOD may be appropriate.
You are currently rated at 100% for IHD, and 50% for PTSD. (Standard VA tactic to avoid SMC payments)
Your letter from the cardiologist actually connected the two, but this will not result in a compensation increase, unless you have 60% beyond the 100% rating.
SSA and the VA do things differently. Even so, the SSA determination of total disability is evidence of unemployable status, regardless of the causes. It could be used if you were asking for a VA pension.
Since you are receiving 100% VA schedular compensation, the VA pension is moot. You are also receiving either SSDI, or if over retirement age, your SSA benefits.

It seems that you, along with hundreds of thousands of other veterans, consider the 100% VA compensation amount inadequate. I happen to agree.
Unfortunately, Congress does not, or they would have increased the compensation, changed the regs/laws, etc. Fat chance of that happening anytime soon.
Currently, the 100% compensation rate is more or less at least $10,000 below what it should be, using the governments own statistics.
(Using average income data, etc.)

A concern you should at least think about is the VA fiduciary program. A high PTSD rating may be considered as evidence against you. Your psychiatrist(s)
need to state in writing that you are competent to manage your own financial affairs to help avoid the fiduciary hassle.

Given the increase involved, and presumably a small amount of retro, it's not too likely that a lawyer wants to take the case on.